Integrated circuits, including computer chips, are manufactured by building up layers of circuits on the front side of silicon wafers. An extremely high degree of wafer flatness and layer flatness is required during the manufacturing process. Chemical mechanical planarization (CMP) is a process used during device manufacturing to flatten wafers and the layers built-up on wafers to the necessary degree of flatness.
Chemical mechanical planarization is a process involving polishing of a wafer with a polishing pad combined with the chemical and physical action of a slurry pumped onto the pad. The wafer is held by a wafer carrier, with the backside of the wafer facing the wafer carrier and the front side of the wafer facing a polishing pad. The polishing pad is held on a platen, which is usually disposed beneath the wafer carrier. Both the wafer carrier and the platen are rotated so that the polishing pad polishes the front side of the wafer. A slurry of selected chemicals and abrasives is pumped onto the pad to affect the desired type and amount of polishing. (CMP polishing is therefore achieved by a combination of chemical softener and physical down force that removes material from the wafer or wafer layer.) Using this process a thin layer of material is removed from the front side of the wafer or wafer layer. The layer may be a layer of oxide grown or deposited on the wafer or a layer of metal deposited on the wafer. The removal of the thin layer of material is accomplished so as to reduce surface variations on the wafer. Thus, the wafer and layers built-up on the wafer are very flat and/or uniform after the process is complete. Typically, more layers are added and the chemical mechanical planarization process repeated to build complete integrated circuit chips on the wafer surface.
Many wafer carriers are provided with a gimbal mechanism or pivot mechanism in order to improve the uniformity of wafer film removal and to improve planarity. The pivot mechanism allows the wafer (and a pressure plate) to tilt, wobble, gimbal, or pivot within the wafer carrier. Thus, the surface of the wafer will remain flush with the polishing pad during rotational polishing, despite misalignment of various parts of the polishing apparatus. Misalignment can occur in the platen, the spindle shaft, the wafer carrier, the table and other parts of the polishing apparatus. In addition, the wafer can be misaligned on the carrier and surface variations present on the polishing pad can cause the wafer not to be completely parallel to the pad. However, the pivot mechanism on the carrier allows the wafer to remain parallel with respect to the pad. Because the wafer remains parallel with respect to the pad, the pivot mechanism allows a predictable amount of film to be removed during polishing.
Pivot mechanisms have been proposed for wafer carriers. An example of a pivoted wafer carrier is shown in Kim et al., Workpiece Carrier with Monopiece Pressure Plate and Low Pivot point, U.S. Pat. No. 5,989,104 (Nov. 23, 1999). Kim shows a gimbal mechanism comprising nested bearing rings that rotate relative to each other via pins disposed in the bearing rings. Aaron et al., Wafer Carrier Rotating Head Assembly for Chemical-Mechanical Polishing Apparatus, U.S. Pat. No. 5,868,609 (Feb. 9, 1999) shows a carrier head that pivots around a central sphere. Hudson et al., Wafer Backing Member for Mechanical and Chemical-Mechanical Planarization of Substrates, U.S. Pat. No. 5,830,806 (Nov. 3, 1998) also shows a wafer carrier that pivots around a central sphere. Sinclair et al., Wafer Carrier For Chemical Mechanical Planarization Polishing, U.S. Pat. No. 6,494,769 (Dec. 17, 2002) shows a wafer carrier in which the entire carrier pivots around the chuck of the carrier. Perlov et al., Carrier Head with a Flexible Membrane, U.S. Pat. No. 6,506,104 (Jan. 14, 2003) shows a wafer carrier having a pivot mechanism comprising a series of ball bearings disposed in a retainer surrounding the base.
A problem remains, however, in that the actual pivoting motion in such wafer carriers is not as smooth or continuous as possible due to friction within the pivoting mechanism. Internal friction causes a lag in the ability of the pivot mechanism to keep the carrier in alignment with the pad. (Friction causes moving components to chatter, or to start and stop at high frequency, thereby causing the lag in the ability of the pivot mechanism to align continuously with the pad.) Thus, portions of the wafer will be pushed either too much or too little against the polishing pad. This results in non-uniform film removal, and hence results in variations in flatness or uniformity of the wafer or layer. These variations can be significant given the extreme tolerances (fractions of a micron in some cases) required during wafer or chip manufacturing.